[MS-ABI] Update to vtordisp computation

A portion of the vtordisp computation that was previously unguarded by a 
test for the declaration of user defined constructors/destructors was 
erroniously adding vtordisps to things that shouldn't have them.  This 
patch correctly guards that codepath.  In addition, it updates the 
comments to make them more clear.  Test case is included.

llvm-svn: 206077